The discovery of gas-phase fluidized bed and stirred reactor processes for the production of polymers, especially polyolefin polymers, made it possible to produce a wide variety of new polymers with highly desirable and improved properties. These gas-phase processes, especially the gas fluidized bed process, provided a means for producing polymers with a drastic reduction in capital investment expense and dramatic savings in energy usage and operating costs as compared to other then conventional polymerization processes.
In a conventional gas fluidized bed process a gaseous stream containing one or more monomers is passed into a fluidized bed reactor containing a bed of growing polymer particles in a polymerization zone, while continuously or intermittently introducing a polymerization catalyst into the polymerization zone. The desired polymer product is withdrawn from the polymerization zone, degassed, stabilized and packaged for shipment, all by well known techniques. Most polymerization reactions, e.g., polymerization of olefins, are exothermic, and substantial heat is generated in the polymerization zone which must be removed to prevent the polymer particles from overheating and fusing together. This is accomplished by continuously removing unreacted hot gases from the polymerization zone and replacing them with cooler gases. The hot gases removed from the polymerization zone are compressed, cooled in a heat exchanger, supplemented by additional amounts of monomer to replace monomer polymerized and removed from the reaction zone and then recycled into the bottom of the reactor. Cooling of the recycled gases is accomplished in one or more heat exchanger stages. The sequence of compression and cooling is a matter of design choice but it is usually preferable to provide for compression of the hot gases prior to cooling. The rate of gas flow into and through the reactor is maintained at a level such that the bed of polymer particles is maintained in a fluidized condition. The production of polymer in a stirred bed reactor is very similar, differing primarily in the use of mechanical stirring means to assist an upwardly flowing stream of gases in maintaining the polymer bed in a fluidized condition.
Conventional gas phase fluidized bed resin production is very well known in the art as shown, for example, by the disclosures appearing in U.S. Pat. Nos. 4,379,758, 4,383,095 and 4,876,320, which are incorporated herein be reference.
The production of polymeric substances in gas phase stirred reactors is also well known in the art as exemplified by the process and equipment descriptions appearing in U.S. Pat. No. 3,256,263.
For many years it was erroneously believed that to allow liquid of any kind to enter into the polymerization region of a gas phase reactor would inevitably lead to agglomeration of resin particles, formation of large polymer chunks and ultimately complete reactor shut-down. This concern caused gas phase polymer producers to carefully avoid cooling the recycle gas stream entering the reactor to a temperature below the condensation temperature of any of the monomers employed in the polymerization reaction.
Comonomers such as hexene-1, 4-methyl pentene and octene-1, are particularly valuable for producing ethylene copolymers. These higher alpha olefins have relatively high condensation temperatures. Due to the apprehension that liquid monomers in the polymerization zone would lead to agglomeration, chunking and ultimately shut down the reactor, production rates which depend upon the rate at which heat is removed from the polymerization zone, were severely constrained by the perceived need to maintain the temperature of the cycle gas stream entering the reactor at temperature safely above the condensation temperature of the highest boiling monomer present in the cycle gas stream.
Even in the case of polymerization reactions conducted in fluidized, stirred reactors, care was exercised to maintain the resin bed temperature above the condensation temperature of the recycle gas stream components.
To maximum heat removal it was not unusual to spray or inject liquid or onto the polymer bed where it would immediately flash into a gaseous state by exposure to the hotter recycle gas stream. A limited amount of additional cooling was achieved by this technique by the Joule-Thompson effect but without ever cooling the recycle gas stream to a level where condensation might occur. This approach typically involved the laborious and energy wasting approach of separately cooling a portion of the cycle gas stream to obtain liquid monomer for storage and subsequent separate introduction into or onto the polymerization bed. Examples of this procedure are found in U.S. Pat. Nos. 3,254,070; 3,300,457; 3,652,527 and 4,012,573.
It was discovered later, contrary to the long held belief that the presence of liquid in the cycle gas stream would lead to agglomeration and reactor shut-down, that it is indeed possible to cool the entire cycle gas stream to a temperature where condensation of significant amounts of monomer would occur without the expected dire results when these liquids were introduced into the reactor substantially in temperature equilibrium with the recycle gas stream. Cooling the entire cycle gas stream produces a two-phase gas-liquid mixture in temperature equilibrium with each other so that the liquid contained in the gas stream does not immediately flash into vapor. Instead, a substantially greater amount of cooling than previously thought possible takes place because the total mass of both gas and liquid enters the polymerization zone at a temperature substantially lower than the polymerization zone. This process led to substantial improvements in the yield of polymers produced in the gas phase, especially where comonomers which can condensate at the temperatures of the polymerization zone, are used. This procedure, commonly referred to as "condensing mode" operation, is described in detail in U.S. Pat. Nos. 4,543,399 and 4,588,790 which are incorporated by reference.
In condensing mode operation, the two-phase gas-liquid mixture entering the polymerization zone is heated quite rapidly and is completely vaporized within very short distance after entry into the polymerization zone. Even in the largest commercial reactors, soon after entry into the polymerization zone all liquid has been vaporized and the temperature of the then totally gaseous cycle gas stream raised, by the exothermic nature of the polymerization reaction. The ability to operate a gas phase reactor in condensing mode was believed possible due to the rapid heating of the two-phase gas liquid stream entering the reactor coupled with efficient constant back mixing of the fluidized bed leaving no liquid present in the polymer bed more than a short distance above the entry level of the two-phase gas-liquid recycle stream.
Commercial polymerization operations have used for years relatively high levels of condensate in the recycle streams, in many instances in excess of 20 weight percent liquid was contained in the recycle stream but always above, the dew point for components in the polymerization zone to assure quick volatilization of the liquid.
While fluidized bed polymerization processes have found particular advantage in the manufacture of polyolefins, the types of polymerization catalysts have been limited to those which are operable in the gas phase. Consequently, catalysts that exhibit activity in solution phase reactions and those which operate by ionic or free radical mechanisms are typically not suitable for in gas phase polymerization processes.